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When Luke Skywalker stared off somberly toward the twin setting suns of his native planet Tatooine in Star Wars, he left some viewers with somber questions of their own. These questions had nothing to do with joining the rebellion. Instead, they were things like, "Could a planet like Tatooine actually exist? Can planets form in a system with more than one star?"

The formation of planets around their stars is a complicated business, and that’s when there’s only one star in the picture. Add two more stars and it can create problems for anything within their gravitational influence.

New observations of the triple star system GG Tau A addresses this issue, confirming theoretical models of planet formation in the process. The new data demonstrates that the system is capable of forming planets and suggests that planets may already be forming in GG Tau A.

GG Tau A

Theoretical models of multiple star systems predict that each star will have its own disk of gas and dust orbiting it. In addition to that, there should be one larger ring, a ‘circumbinary’ disk (assuming there are two stars), orbiting the system. This is observed in GG Tau A, which contains three stars and lies 460 light-years away in the constellation Taurus. The system’s biggest star, GG Tau Aa, has a disk that was imaged in the new study.

Orbiting 35 astronomical units (au) away from Aa (close to the average distance between the Sun and Pluto), the system’s second and third stars, called GG Tau Ab (individually Ab1 and Ab2), orbit each other. Their disks, if they exist, were not observed in this study, though they were inferred from earlier infrared observations.

Another disk was also observed orbiting the system as a whole. Specifically, it surrounds a cavity of empty space between Aa and Ab, a cavity cleared of gas and dust by the stars’ gravity. The disks that we've observed are similar to what we've predicted for the protoplanetary disk around the sun that became the planets of the solar system. But there’s a problem.

Not enough time

It takes a fair bit of time for a planet to form out of the matter in such a disk, about a million years (although, admittedly, that's short in astronomical terms). In our solar system, this was not a problem. But in a triple star system, it’s not easy for the disk material to maintain a stable orbit for very long, given the complex gravitational influences. Even in a stable system, the disk’s material tends to fall into its star, a process which may take only a few thousand years. At that rate, the disk would be depleted long before it could form any planets; at GG Tau Aa, it would be gone by now.

In order for any planet formation to happen, the disk needs to be replenished by an outside source. Luckily, such a source is readily available: the disk surrounding the system.

A key discovery in the new work is the detection of material in the cavity that the outer disk surrounds. Though previous work had implied its presence, the cavity was thought to be empty. The material appears to be a flow from the circumbinary disk to Aa’s smaller disk. This flow, the authors conclude, is enough to keep Aa’s disk replenished for over a million years—long enough for a planet to form.

The discovery of the influx of material explains why Aa’s disk hasn’t decayed long ago, as the system is one to five million years old.

Distant planet

In addition to the finding that Aa’s disk is capable of forming planets, the scientists suspect that a planet may be forming in the outer disk itself. The proto-planet, if it exists, rests at about 250 au from the stars. That’s roughly five times Pluto’s distance from the sun at the most distant point in its orbit. That puts it right in the middle of the outer disk, which runs from 190 to 280 au away from the stars.

The presence of such a body could explain why the disk is relatively compact, with 80 percent of its mass confined within 90 au.

Binary systems are very common, making up at least half of the stars visible in the galaxy. There are also many systems with more than two stars. In all of these systems, planetary formation is a complex business. This work is a step toward understanding some of complexities, toward a fuller understanding of the mechanisms that drive planet formation in a significant fraction of the galaxy.

“Almost half the Sun-like stars were born in binary systems. This means that we have found a mechanism to sustain planet formation that applies to a significant number of stars in the Milky Way. Our observations are a big step forward in truly understanding planet formation,” said Emmanuel Di Folco, the paper's co-author, in a press release.